An emerging class of compact gas discharge lamps employs a standard Edison-type screw base, for installation in a conventional lamp socket that can also accommodate incandescent lamps. Such compact gas discharge lamps comprise low pressure fluorescent lamps that employ a multi-axis envelope, or discharge vessel, in which light is emitted from a suitable fill that is electrically excited to a discharge state. The power supply, or as more conventionally known, "ballast" circuit for such a compact fluorescent lamp typically has an expected lifetime of four or more times that of the mentioned multi-axis envelope and any associated circuitry. It would, therefore, be desirable to provide a gas discharge lamp ballast circuit that includes an indicator of operability of the ballast circuit. In this way, the ballast circuit can be saved for re-use with a removable gas discharge lamp.
As the present inventors have discovered, an indicator of operability of a lamp ballast circuit can beneficially be coupled to the output of a circuit for sensing an overload condition of voltage on terminals for connecting to a lamp (hereinafter, "lamp terminals"). Such sensing circuitry is typically used with a shut-down circuit for terminating the supply of voltage to the lamp terminals. A typical prior art overload sensing circuit, however, suffers from the drawback of only detecting one of the positive or negative excursions of lamp voltage. Such circuit may, therefore, fail to sense excessive excursions of lamp voltage in the other polarity, wherein the lamp is operating in a so-called rectification mode; such mode may occur, for instance, where one of the lamp cathodes has become damaged. Continued operation of the lamp in a rectification mode places above-normal stress on the components of the ballast circuit, and thereby risks reduced ballast life.
It would, therefore, be desirable to provide a gas discharge lamp ballast circuit that includes a circuit for sensing an overload condition of voltage on the lamp terminals, including a rectification mode failure of the lamp.
A gas discharge lamp ballast circuit employing a prior art overload sense circuit, as described above, relates the use of a pair of switches that are alternately switched into a conduction state, for providing bidirectional current to a resonant load circuit including a gas discharge lamp. Control of the switches is typically provided by circuitry that supplies the control terminals, or "gates," of the switches with respective control signals derived from feedback of a current in the resonant load circuit. Control of the switch gates in this manner is known in the art as "self-resonant" gate control.
Typically, a prior art ballast circuit of the foregoing type will incorporate an overload sense circuit, as described above, and a responsive, shut-down circuit for terminating self-resonant gate control, and, hence, potentially damaging current in the switches. A drawback arises where such ballast circuit further uses a start-up circuit of the non-latching latching type for initiating self-resonant gate control by providing a start-up pulse to the gates of the switches. Owing to the non-latching nature of such start-up circuit, such circuit will, shortly after shut down of the switches in response to an overload condition, provide yet another start-up pulse, to thereby re-initiate self-resonant gate control. With an overload condition persisting, the shut-down circuit will again terminate the self-resonant gate control, but the non-latching start-up circuit then re-initiates the self-resonant gate control. This process repeatedly occurs, placing the components of the ballast circuit under abnormally high stress, thus shortening ballast life.
It would, therefore, be desirable to provide a gas discharge lamp ballast circuit that avoids the repeated stressing of ballast circuit components arising from repeated re-initiation of a self-resonant mode of gate control during an overload condition.